1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 Hempel A/S
Self-healing anticorrosive coatings are multi-component so-called smart materials, which have been proposed as a way to long-lasting corrosion protection of steel structures. The presently most promising technology route is based on microcapsules, filled with active healing agents, and has been the focus of this work. The microcapsules consist of a solid polymeric shell and a liquid core material. When a microcrack, originating from internal stress or a physical damage, propagates through the coating, the microcapsules rupture and release healing agents, which flow to the fracture plane due to capillary forces. The healing agents then start to react, form a polymer network, and =glue‘ the crack. The approach has been applied to development of an epoxy-based self-healing anticorrosive coating for above water heavy duty corrosion protection. Emphasis has been on investigation of practical issues associated with development and testing of this type of coating. A laboratory investigation, to identify the most suitable method for production of mechanically stable (filled with industrially relevant core materials) and forming a free-flowing powder upon drying microcapsules, has been performed. Four different experimental procedures, available in the literature, have been used for encapsulation of six core materials, including epoxy resins, diluent, and linseed oil. Several challenges have been identified during the investigations. Main of them dealt with encapsulation of viscous healing agents and a necessity of a thorough adjustment of the synthesis procedures for a wider use with other than original core materials. Free-flowing powders of two types of microcapsules (filled with linseed oil and alkylglycidylether) have been produced and investigated for solvent stability, stability towards stirring and storage, as well as ease of capsule dispersion. A systematic laboratory study, for reduction of poly(urea-formaldehyde) microcapsule size, filled with linseed oil, has been performed. Several synthesis parameters were varied (temperature, stabilizer content, stirring rate, stirrer geometry) and mechanical means of separation were investigated. Capsules with a mean diameter less than 150 µm were obtained using a steel sieve coated with a fluoropolymer coating. These smaller capsules were used in further investigation as model capsules. A range of microcapsule-containing coatings was formulated, applied to steel substrates, and subjected to salt spray exposure and reverse impact testing. Neither of the tests revealed any drawbacks from addition of microcapsules to an epoxy coating in a concentration up to 50 vol %. On the contrary, the results of the impact test has shown that addition of microcapsules reduces the intensity of crack formation (both in number and length) compared to filler-containing coatings and prevents the coating from flaking upon damage. Based on specular gloss measurements, a preliminary critical pigment (microcapsule) concentration (CPVC) value was estimated to about 30 vol %. The number is lower than anticipated and needs to be confirmed. Finally, a 3-D model, based on Monte-Carlo simulations, has been developed for prediction of healing efficiency of a microcapsule-based anticorrosive coating. Two kinds of cracks were considered: cracks accommodated within the bulk coating and cracks starting from the coating surface. The model takes into account volume of the crack formed, crack geometry and linear dimensions, as well as diameter, volume concentration, and wall thickness of the microcapsules embedded in the coating. Simulations showed that diameter of microcapsules and crack geometry played an important role in the self-healing action of the coating, especially when low concentrations of capsules were used.
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Kiil, Søren, Dam-Johansen, Kim, Pedersen, Lars Thorslund